Following the destruction of the Chimera's central tower in London and their armies eradicated throughout Britain in the Battle of London, humanity was aware that the Chimeran tower network was the key source of the Chimera's vulnerability. The Maquis, an European mainland resistance movement, approached the British for a joint-planned operation in destroying the Chimera's hub tower in Paris, France. However, the operation was not only to destroy the Paris central power, but also developing a serum created by Dr. Claude Bouchard that would eradicate the Chimera's Carrier strain that were tasked in transporting Chimeran virus-infected humans to conversion centers, and thus grounding the Chimera's conversion process to a halt.
Operation Overstrike is quite similar to Operation Deliverance in the sense that new troops are being transported to fight and liberate a country (in this case a continent) from the Chimera. The British Intelligence Corps would dispatch two squadrons of British forces to aid the Maquis. The First Squadron under the command of Major Stephen Cartwright would fly first into Rotterdam, in which it is one of the few open corridors to bypass the Chimera's coastal fortifications, follow four hours later by the Second Squadron under Colonel Rachel Parker. Accompanied by the Maquis, the British would cross Chimera lines into Luxembourg where the Maquis stronghold lies, and then from there preparing their attack on the Paris Tower, hoping that its destruction would result in the meltdown of the tower network across Western Europe.
In the early phase of the operation, the Maquis made a major intelligence error in regarding to the Chimera's coastal defenses. Since the Chimera's occupation of Europe, the Chimera constructed massive fortifications on the coastlines of Western Europe and making it almost impossible to penetrate through. The Chimera, however, hastily erected three gun towers in Rotterdam that came unnoticed for the Maquis and inflicting heavy casualties on Major Cartwright's First Squadron.
Despite the losses, human forces were able to disable the gun towers and allowing Parker's Second Squadron to safely arrive, bringing supplies and reinforcements to a makeshift camp outside of Rotterdam. Cartwright would lead his task force to intercept Chimeran Earth Movers in Gerolstein that were heading for Luxembourg while Parker took the remainder of the Squadron to Luxembourg to finalize plans for the assault on the Paris hub tower.
During this time, British mercenary James Grayson, Colonel Roland Mallery, and Lieutenant Raine Bouchard investigated a new type of conversion facility in Bonn, Germany. The group were separated, with Bouchard captured by the Chimera and falling into the initial stages of the conversion process, but Grayson was able to rescue her, believing her to be yet uninfected, and then destroying the facility.
On September 2nd, Major Cartwright faced complications in stopping the Chimeran Earth Movers in Gerolstein. James Grayson aided Cartwright in immobilizing the Earth Movers, but their victory was overshadowed by an all-out Chimeran assault on the Maquis Fortress.
British and Maquis soldiers defended the fortress long enough to evacuate to the Maquis's last remaining safe zone in Reims. Dr. Claude Bouchard along with his daughter Raine were still occupied in developing the Carrier-killing serum, in which Raine had deliberately allowed herself to being infected in Bonn to allow her father to gain a sample of the Chimeran virus. However, before the serum could be created, Dr. Bouchard was killed by the Chimera, but Raine was able to finish her father's work and gaining a sample. All surviving human personnel and refugees fled to Reims.
Following the evacuation to Reims, plans for the assault on the Paris tower were laid: Cartwright would lead the combined human military forces into battle. However, their goal is not only to destroy the hub tower, but also to distract the Chimera while allowing an infiltration team into the Chrysalis Lair, a conversion facility where Chimera DNA are implanted into captured humans before they are taken to other conversion facilities, and releasing Dr. Bouchard's serum. Raine Bouchard volunteered for the infiltration, but was in fact already infected with the Chimeran virus and showing signs of falling to the Chimera's influence. James Grayson, having discovered Bouchard's infection, attempted to warn the Maquis and the British about Bouchard, but only to be refuted by Mallery and imprisoned for treason.
Fortunately, Grayson was freed by Cartwright and Rachel Parker and given a second sample of the serum to take to the Chrysalis Lair through the Paris catacombs. At the same time Mallery's forces engaged the Chimera in the city and suffering a grave amount of human casualties. Grayson encountered Mallery, who was became infected with the Chimeran virus and purposefully preventing Grayson from reaching the facility. Grayson killed Mallery and continued into the Chrysalis Lair, where he succeeded in using the serum just as Cartwright's forces destroyed the Paris tower. Just after Grayson used the serum, he discovered that Bouchard had mutated into a Chimeran entity calling itself the Chrysalis and managed to kill it.
Operation Overstrike resulted in a major victory for humanity. Just as the tower fell in London, the Chimeran tower in Paris was destroyed by Major Cartwright and his task force, freeing Western Europe from the Chimera. Claude Bouchard's serum eradicated the Carrier strains and stopping the conversion process. Poland, Spain, the Balkans and Russia were still occupied by the Chimera, but after Overstrike these countries became the main battlefields in Northern, Eastern and Southern Europe.
Despite the eradication of the Carrier strain, the Chimera were able to overcome their setback by evolving and creating new strains among their ranks. Among them that succeed the Carriers was the Spinner strain. Furthermore, the Chimera finished the construction of a mass fleet of airships with their targets and destinations unknown at the time. On September 9th, the British and Maquis built a memorial for those who died in the war in Europe, and Grayson was promoted to Major.
- In Resistance: The Gathering Storm, Daedalus was looking through the eyes of other Chimeran species, in which he looked through the eyes of a Steelhead eating a human leg in Paris even though earlier Noah Grace was congratulating Stephen Cartwright for freeing western Europe. This can either be a mistake by the author or the Chimera trying to retake their lost territories like in Bracknell and Axbridge depicted in Resistance 2.